1. Field of the Invention
The present invention relates generally to restorative dentistry and more particularly to torque wrenches for dental components.
1. Background Information
The use of dental restorations to replace missing teeth and techniques for such use are well known in the prior art. One of these techniques requires that one or more implants be affixed to either the maxillary or the mandibular bone of a patient. Such implants are typically cylindrical and are internally screw threaded. A casting is made which fits over one or more implants. The casting is held in place by a screw which passes through the casting and screws into the threaded portion of the implant. In many cases two such screws are necessary, because the casting is attached to two implants. Castings typically include some type of fastening system for attachment of dental prostheses. Such a screw typically includes an internal hex opening or slot in the end opposite the threads. A driver with either a hex head or blade matching the opening or slot in the screw is typically used to tighten the screw.
A problem is always encountered in the technique of using implants and castings as described above: applying the correct torque on the driver to provide the appropriate screw tightness. If the screw or screws holding the casting are too loose, the dental prosthesis will be loose or may become displaced. If too much torque is applied to a screw, the screw may be fractured or weakened to the point that it will fail in the future. In those cases in which one casting is attached to two or more implants, it is also very important that the same torque be applied to each of the attaching screws to prevent undue pressure on any of the implants. A similar problem encountered when one casting is attached to two or more implants is that the fit of the casting must be passive. That is, even though the screws attaching the casting may be tightened to the same torque, the screws, implants, and the casting will be subjected to unacceptable stress if the casting is bent or twisted after it is installed. A passive fit may be assured by adjusting the casting so that each of the screws is tightened to the appropriate torque after being turned approximately the same number of revolutions or part of a revolution from being finger tight.
Several attempts have been made to solve the aforementioned problems. One such attempt is disclosed in the patent to Deutsch et al. (U.S. Pat. No. 4,480,997, Nov. 6, 1984). The Deutsch patent discloses a wrench used to apply a predetermined torque to a dental post. The wrench has a cylindrical casing designed to be turned with the thumb and index finger. A clutch inside the casing may be adjusted so that the wrench slips when a predetermined torque is reached.
Another attempt to solve problems related to dental screw torque is disclosed in the patent to Perry (U.S. Pat. No. 5,158,458, Oct. 27, 1992). The device disclosed in Perry includes a driver portion which engages the screw and a mandrel to which the turning force is applied. A torque element is interposed between the driver and the mandrel which engages with each of them. The torque element includes an inner member which engages the mandrel and an outer member which engages the driver. The inner and outer members are connected by spokes. When the torque between the driver and the mandrel reaches a predetermined level the spokes break indicating that a particular torque has been applied to the screw.
Another attempt to solve problems related to dental screw torque is disclosed in the patent to Patterson et al. (U.S. Pat. No. 5,295,831, Mar. 22, 1994). The device disclosed in Patterson is essentially an Allen wrench with a calculated weak spot built into one of the legs. When a screw is being tightened using the wrench and a predetermined torque is reached, the weakened spot gives way and the wrench deforms.
Another problem associated with the use of torque wrenches for tightening a dental component is the necessity for sterilization. Only sterile dental implements and devices should be introduced into a patient's mouth
Another problem associated with the use of torque wrenches for dental components is the occasional necessity to determine the torque required to loosen a screw which attaches a dental component. This may be necessary, for example, when a single casting is affixed to two implants by means of two screws and the screws should be tightened to the same torque.
All of the known dental wrenches require either that a different wrench or element be chosen or that an adjustment be made to the wrench to measure differing toques. Several of the know dental wrenches may only be used once or require replacement of an element with each use. Other dental wrenches require a variety of relatively complicated moving or interacting parts or are difficult to clean or sterilize. The ideal wrench for applying a measured torque to a dental component is compact, lightweight, easy to use, usable for many different torque requirements, and has few, if any, moving or interacting parts. Another requirement of the ideal dental torque wrench is that it be easily cleaned and sterilized. Another requirement of the ideal dental torque wrench is that it may be used to determine the torque necessary to loosen the screw on a dental component as well as the torque necessary to tighten such a screw.